I have always been under the assumption that Lange's metal choice, or lack thereof, was based off of production capacity and price point. For example, I would pose that they discontinued the Datograph Perpetual in Platinum because of production capacity, and they chose to make the new 1815 Up/Down only in the three golds because of price point.

I also think that they are trying to raise the "value" of a piece that they make in platinum and keep the specific model family more desirable and rare; examples being the new Datograph Up/Down or the Lumen.

Nate

p.s. after having tried on all three metals of the 1815 Up/Down, the yellow gold was by far my favorite.